Applicant claims priority of PCT application Ser. No. PCT/GB98/01868; filed May 28, 1998.
Not applicable.
1. Field of the Invention
This invention relates to a spectrum analyser.
2. Description of the Related Art
An ideal spectrum analyser is illustrated in FIG. 1 of the accompanying drawings. A control processor 1 tunes the bandpass filter 2 such that it passes only a selected range of the frequencies present in input signal 3. The power of this signal is detected by power detector 4. The basic output of the power detector 4 is then processed by processor 5, where the signal may be averaged to reduce noise, peak detected, and converted to a logarithmic representation, before being displayed on a display device 6. Usually, the control processor 1 will organise the display such that a graph is drawn of power versus frequency.
An RF spectrum analyser of this simple form is impractical, certainly if intended as a general purpose test instrument. The problem lies in the implementation of the tuneable filter 2. It is impractical to make a filter that has a bandwidth which is selectably wide or narrow (e.g. 3 MHz to 10 Hz) that will tune over a large RF frequency range (e.g. 10 kHz to 10 GHz). An interesting contrast to this is the optical spectrum analyser, where a cavity resonator and/or a diffraction grating can be tuned over the full range of interest, and the heterodyne techniques described to follow are not practical.
A solution to the implementation problem is found with a frequency converting front end to the spectrum analyser. FIG. 2 of the accompanying drawings shows the use of such a device. Instead of tuning a bandpass filter on the input, the control processor 1 sets the frequency of a frequency synthesiser 7, which provides a frequency reference to a frequency converter 8. The function of the frequency converter 8 is to take a block of frequencies, which frequencies are related to the reference frequency, and to convert them, maintaining their relative power, into a block of frequencies with the same range, but a much lower centre frequency. This block of frequencies is passed to a fixed bandpass filter 9, where one narrow range is selected and passed on for power detection by the power detector 4.
The frequency conversion stage 8, combined with the reference synthesiser 7 and the fixed bandpass filter 9 emulates the bandpass filter 2 of FIG. 1, but frequency shifted up in frequency by an amount related to the frequency reference. As it is implemented at a fixed and low frequency, the bandpass filter 9 of FIG. 2 is possible to realise.
An ideal frequency conversion stage for use in this type of spectrum analyser has one important property, which will be called the 1:1 property: whenever signal power emerges from the frequency converter, there is always exactly one input signal at a specific frequency difference and power difference on the input signal that has caused it. The specific power difference allows the controller 1 to correctly estimate the input power that it has detected. The specific frequency difference allows the controller 1 to correctly estimate the frequency of the signal that it has detected.
It is not easy to build an ideal frequency converter. Referring to FIG. 3 of the accompanying drawings, the simplest type uses a fundamental mixer 10. Consider an input signal of 1 GHz, and a reference signal of 1.01 GHz. The frequency mixer 10 will output two signals of frequencies 2.01 GHz, and 10 MHz. The bandpass filter 9 following the mixer 10 will reject the 2.01 GHz signal, and pass the 10 MHz signal. Unfortunately, there are other input signals that can cause an output of 10 MHz. The first and most obvious is an input signal of 1.02 GHz, which being different from the reference by 10 MHz, will cause a 10 MHz output signal. In addition, non-idealities in the mixer 10 will cause input signals such as 3.02 GHz, which is 10 MHz different from the third harmonic of the reference to cause an output of 10 MHz. Thus, it can be seen that the fundamental mixer violates the 1:1 property, of only one input signal causing an output. The result of this violation would be that the spectrum analyser display would indicate the presence of input power where there was none, causing xe2x80x98imagesxe2x80x99 and xe2x80x98spurious signalsxe2x80x99 on the display.
There are instruments that can be built using a fundamental mixer. A modulation analyser is one such instrument. Here the assumption is made that in normal operation of the instrument, the only input signal is the modulation under test. Though the simple RF front end is capable of creating images and spurious signals, these are known to be absent under normal use, and so are ignored.
A spectrum analyser cannot make the assumption of a single input signal. A measurement that is often made with a spectrum analyser is a search for signals. Here any signal seen on the display must be able to be interpreted as a genuine input signal, and not ignored as an artefact of the frequency converter.
Referring to FIG. 4 of the accompanying drawings, a more ideal frequency converter can be built with multiple mixers and filters. A low pass filter 11 passes only signals below some cut-off frequency, for instance 2 GHz. Therefore signals only in the range DC to 2 GHz are present on the mixer input. The reference synthesiser 7 generates frequencies higher than this range, for instance 3 GHz to 5 GHz, and the bandpass filter 12 is tuned to 3 GHz. This ensures that when the reference synthesiser 7 is set to a frequency of 4 GHz, only input signals with a frequency of 1 GHz will appear in the bandpass filter 12. The input lowpass filter 11 ensures that there will not be images caused by a 7 GHz input mixing with the 4 GHz reference to create a mixer output frequency also of 3 GHz. This describes the input stage of the classic xe2x80x98upconversionxe2x80x99 heterodyne receiver. While a power detector could be placed at the output of the bandpass filter 12, it is very difficult to make a narrow bandwidth filter at such a high frequency, and so one or more further stages of mixing and filtering are employed to get the final signal down to a reasonable frequency.
A second reference source 13 is used together with a second mixer 21. If the reference is chosen to be 3.01 GHz, then the output frequency in the final bandpass filter 9 will be 10 MHz. However, there are practical problems with these particular frequencies. If bandpass filter 12 contains a signal at 3.02 GHz, it too will mix down to an output of 10 MHz. It is not easy to make the bandpass filter 12 have a passband centred on 3 GHz, and also to provide adequate rejection of signals at 3.02 GHz. Without adequate rejection of these image frequencies, the complete frequency converter will violate the important 1:1 property, even though the first mixing stage does not. The best that can be routinely obtained from a typical filter at 3 GHz is to reject signals 300 MHz away from the passband. The second reference 13 must be offset more from 3 GHz, perhaps to 3160 MHz, such that the following bandpass filter 9 is centred on 160 MHz.
This final output frequency may still be too high to build narrow resolution filters. However, it is now low enough to build a filter which will discriminate against images at 10 MHz away. A further stage of mixing and filtering can now be employed to bring the final output frequency down to 10 MHz, while retaining the 1:1 property.
In the classical analogue spectrum analyser (FIG. 2), filter 9 is switch selectable between several resolution bandwidths, perhaps 100 Hz to 1 MHz, and defines the resolution bandwidth of the instrument. The power in this filter is detected by power detector 4, and represents the amount of power present at the tuned frequency. The frequency reference 7 to the frequency converter 8 is stepped or swept across a range of frequencies in order to build up a picture of power versus frequency on the display 6.
An alternative to this analogue back end is shown in FIG. 5 of the accompanying drawings. Here bandpass filter 9 serves not to define the resolution bandwidth of the instrument, but only to define the set of frequencies coming from the frequency converter 8, and entering the low frequency spectrum analyser 14. Typically, the low frequency spectrum analyser 14 will be implemented as a high speed analogue to digital converter, followed by a digital signal processor (DSP) which implements a discrete Fourier transform (DFT). While all analogue spectrum analysers still have an advantage on absolute dynamic range performance (typically  greater than 80 dB), the digital back end is becoming more popular, especially due to its speed when implementing narrow resolution bandwidths (typically  less than 10 kHz). The result of the DFT is power readings for a range of frequencies. If this range is not sufficient, the frequency reference may be stepped and a fresh set of readings made. The blocks of readings are then xe2x80x98stitchedxe2x80x99 together to make a contiguous display.
The foregoing description, excepting the details of the frequencies and the exact number of stages, describes the RF front ends of the great majority of commercial RF spectrum analysers produced. While the standard heterodyne system can be made to work well, it has a number of disadvantages. Many stages cost more, use more power, and take up more space, than a single stage. Many stages provide more opportunity for level inaccuracies to accumulate, and for the generation of high order spurious signals in and between the many mixers. The frequencies employed within the instrument are well above the highest input frequency measured. The use of an input lowpass filter, with all of the frequency references related to it, makes for a relatively inflexible structure.
According to the present invention there is provided a spectrum analyser comprising: means for converting at each of a series of frequency settings thereof a received radio frequency signal into an intermediate frequency signal, each said intermediate frequency signal produced being derivable from more than one nominal said received radio frequency signal; means for carrying out a frequency analysis of each intermediate frequency signal to produce a power spectrum thereof; means for constructing a composite received radio frequency signal power spectrum corresponding to each said intermediate frequency signal power spectrum, said means for constructing comprising: means for determining in respect of each frequency interval of the intermediate frequency signal power spectrum which of frequency intervals of the corresponding radio frequency signal power spectrum could have given rise to the presence of a power level at that frequency interval of the intermediate frequency signal power spectrum; and means for assigning a corresponding power level to the or each said determined frequency interval of the radio frequency signal power spectrum; and means for operating on the constructed composite radio frequency signal power spectrums to provide the actual power spectrum of said received radio frequency signal.
Preferably, said means for converting comprises: a frequency synthesiser for synthesising the frequencies of said frequency settings; and a harmonic mixer for mixing each synthesised frequency with the received radio frequency signal, the nominal received radio frequency signals corresponding to each intermediate frequency signal thereby equalling N.Frefxc2x1IF, where N ranges over a number of integers, Fref is the synthesised frequency, and IF the intermediate frequency. Alternatively, preferably, said means for converting comprises: a frequency synthesiser for synthesising the frequencies of said frequency settings; and a fundamental mixer for mixing each synthesised frequency with the received radio frequency signal, the nominal received radio frequency signals corresponding to each intermediate frequency signal thereby equalling Frefxc2x1IF, where Fref is the synthesised frequency, and IF the intermediate frequency.
Preferably, said means for converting further comprises: a low pass filter for filtering the received radio frequency signal before it is passed to said mixer, said low pass filter thereby defining the upper frequency limit of the spectrum analyser; and a band pass filter for filtering the output of said mixer to provide said intermediate frequency signal, said band pass filter thereby defining the range of frequencies supplied to said means for carrying out a frequency analysis.
Preferably, said means for carrying out a frequency analysis comprises: means for digitising each intermediate frequency signal; and Fourier transform means for Fourier transforming each digitised intermediate frequency signal to provide its power spectrum.
In the analyser of the preceding paragraph, preferably, said means for constructing and said means for operating together comprise a control processor and memory means, said Fourier transform means storing each intermediate frequency signal power spectrum in said memory means, said control processor addressing said memory means to determine that determined by said means for determining, said control processor assigning the corresponding power levels assigned by said means for assigning and storing these power levels in said memory means thereby to construct in said memory means said composite radio frequency signal power spectrums, said control processor addressing said memory means to operate on the composite radio frequency signal power spectrums to provide the actual power spectrum of the received radio frequency signal, said control processor storing said actual power spectrum in said memory means.
In the analyser of the preceding paragraph, preferably, there is a predetermined limit to the number of composite radio frequency signal power spectrums permitted to be stored in said memory means, and once said limit has been reached said control processor examines the power levels in corresponding frequency intervals of the stored composite radio frequency signal power spectrums to determine whether a stored power level should be replaced by a current power level, and if so, which one should be replaced. Alternatively, in the analyser of the preceding paragraph, preferably, said processor means maintains a single said stored composite radio frequency signal power spectrum in said memory means, which single spectrum it continually updates in dependence on power levels subsequently assigned by said means for assigning, the final stored composite radio frequency signal power spectrum thereby being the said actual power spectrum of the received radio frequency signal.
As an alternative to the aforementioned analyser wherein said means for carrying out a frequency analysis is digital in form, preferably, said means for carrying out a frequency analysis comprises one or more band pass filters, and a power detector following the or each band pass filter.
Preferably, said means for carrying out a frequency analysis performs an initial frequency conversion of each said intermediate frequency signal.
Preferably, said means for operating determines the minimum power level present in corresponding frequency intervals of the constructed composite radio frequency signal power spectrums, and provides as the actual power spectrum of the received radio frequency signal the power spectrum comprising the determined minimum power levels. Alternatively, preferably, said means for operating determines the median of the power levels present in corresponding frequency intervals of the constructed composite radio frequency signal power spectrums, and provides as the actual power spectrum of the received radio frequency signal the power spectrum comprising the determined median power levels. Alternatively, preferably, said means for operating carries out a robust mean estimation process on the power levels present in corresponding frequency intervals of the constructed composite radio frequency signal power spectrums, and provides as the actual power spectrum of the received radio frequency signal the power spectrum comprising the power levels resulting from the robust mean estimations.
Preferably, in order to analyse signals the components of which vary with time, the following characteristics of said means for converting are selectably adjustable: the absolute times at which said frequency settings are adopted by said means for converting; the length of time for which said means for converting remains at a said frequency setting; the frequency difference between adjacent said frequency settings; and the order in which said frequency settings are adopted by said means for converting.
The present invention provides a spectrum analyser that can use simple RF front ends which do not meet the 1:1 property, for instance a fundamental mixer, a downconversion front end that converts to the final frequency in a single stage, or a harmonic mixer (sampling gate). Each of these front ends has a different cost/performance trade-off with the others, and with respect to the full conventional spectrum analyser front end.